1. Field of the Invention
The invention relates, generally, to a web severer and more specifically, to a speed enhanced full force web severer.
2. Description of the Related Art
In current web severer configurations, an air cylinder is used to move a severing knife into and through a web or ribbon in a folder. To accomplish this act, the air pressure must be evacuated from one side of the air cylinder and introduced into the other side of the air cylinder. In doing so, the knife begins to move as the pressure builds up in the air cylinder.
The length of time required to sever the web is most critical. Since printing presses operate at speeds in excess of 15.25 meters per second, the severer must complete its function in a fraction of a second. One of the time critical limitations in the severing process is the time necessary to build up to maximum air pressure in the air cylinder. The pressure build up time thus controls the severing time. In addition, most severer assemblies begin to move as soon as air is introduced into the air cylinder (i.e. 5 pounds), and the knife contacts the web before the air cylinder reaches maximum air pressure. When the knife hits the web before achieving full force, the web can drag on the knife before the web is severed. Samples taken from various folders have exhibited several feet of knife dragging marks before the knife severed the web.
Attempts to eliminate these deficiencies have led to many innovations. It is common to add a quick dump valve to the appropriate side of the air cylinder to evacuate air faster during the cutting process. This allows the cutting member to start its acceleration sooner. Another solution involves a reduction in the web severer assembly mass that allows for faster acceleration of the knife. A further solution involves the installation of an air reservoir close to the air cylinder for allowing a faster pressure build up in the air cylinder. Still another solution involves the installation of a valve with a high Cv value that results in a faster build up in the air cylinder. A still further solution involves the installation of air lines having large air lines. The larger air lines allows a faster pressure build up in the air cylinder. Finally, optimization of the angle at which the knife attempts to shear the web has led to more efficient severing. All of these enhancements have led to incremental improvements in web severing. However, the ever increasing web speeds now require even faster severing times than is available with the above mentioned improvements by themselves or in combination.
It is accordingly an object of the invention to provide a full force web severer which overcome the herein-mentioned disadvantages of the heretofore-known devices and methods of this general type, in which the time for severing webs is reduced and the severer contacts the web with the maximum available force.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for severing a web, which includes:
a) providing a first given pressure to a first side of an air pressure cylinder driving a cutting member;
b) providing a second given pressure to a second side of the air pressure cylinder that is greater than the first given pressure during a non-cutting state of the air cylinder, the cutting member remaining stationary as long as the second given pressure being greater than the first given pressure; and
c) reducing rapidly the second given pressure to below the first given pressure during a cutting state of the air pressure cylinder for moving the cutting member to sever a web.
In accordance with an added feature of the invention, there is the step of repeating steps b) and c) for each severing operation.
In accordance with another feature of the invention, there is the step of providing a control unit outputting control signals received by the air cylinder for controlling the first given pressure and the second given pressure and for transitioning between the cutting state and the non-cutting state.
In accordance with an addition feature of the invention, there are the steps of: using a first valve for providing the first given pressure and a second valve for providing the second given pressure; and controlling the first valve and the second valve via the control signals output by the control unit.
In accordance with a further added feature of the invention, there is the step of providing three-way valves as the first valve and the second valve.
In accordance with a further additional feature of the invention, the first given pressure is in a range of 70-75 psi and the second given pressure is greater than or equal to 90 psi in the non-cutting state.
In accordance with a concomitant feature of the invention, there is the step of using a quick exhaust valve for rapidly reducing the second given pressure during step c). The pressure is preferably reduced to near 0 psi as soon as possible.
Other characteristic features of the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a full force web severer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.